Packets are not arrived at tunnel setup by libgtpnl

Hi guys, I'm using libgtpnl tools (gtp-link, gtp-tunnel) to setup a tunnel in my local machines. I was able to setup gtp link to 2152 port as well as gtp tunnel. I have pre-recorded GTP packets with correct teid and IP address as well as chksum and port to send to 2152. However I can not see any packets in gtp link device when using tcpdump. By add some messages in gtp module, I notice that the function gtp1u_udp_encap_recv in gtp.c kernel file was never been called if I send packet with GTP headers. However, that function is called if I send normal UDP packet without GTP header. Note that I'm using Ubuntu 14.04 with kernel 4.8.0 lowlatency, the tunnel is on a VM created by Virtualbox using virtio network driver. Any suggestions on how to debug/solve this problem? Many thanks in advances. Best regards Cong.